The present disclosure relates generally to spherically mounted retroreflectors that include an embedded cube-corner retroreflector.
A laser tracker instrument measures the coordinates of a point by sending a laser beam to a retroreflector target in contact with the point. The instrument determines the coordinates of the point by measuring the distance and the two angles to the target. The distance is measured with a distance-measuring device such as an absolute distance meter (ADM) or an interferometer (IFM). The angles are measured with an angle-measuring device such as an angular encoder. A gimbaled beam-steering mechanism within the instrument directs the laser beam to the point of interest.
Ordinarily the laser tracker sends a laser beam to a retroreflector target. A common type of retroreflector target is the spherically mounted retroreflector (SMR), which includes a cube-corner retroreflector embedded within a metal sphere. The cube-corner retroreflector includes three mutually perpendicular mirrors. The vertex, which is the common point of intersection of the three mirrors, is located near the center of the sphere. Because of this placement of the cube corner within the sphere, the perpendicular distance from the vertex to any surface on which the SMR rests remains nearly constant, even as the SMR is rotated. Consequently, the laser tracker can measure the 3D coordinates of a surface by following the position of an SMR as it is moved over and held in contact with the surface. Stating this another way, the laser tracker needs to measure only three degrees of freedom (one radial distance and two angles) to fully characterize the 3D coordinates of a surface.
The most precise applications are pushing for every micron of accuracy possible so every element of the SMR is important. The most accurate SMR models require the highest precision and quality spheres, near perfect geometry and clarity of the optics, assembled with processes that hold mere microns as tolerances. These state of the art opto-mechanical assemblies are verified by high performance instruments in temperature controlled rooms to confirm that design criteria are fully met. When combined with modern laser trackers, extreme accuracy and range are achievable.
Presently, there are three basic configurations of retroreflectors in SMRs: a solid glass retroreflector, an open-air retroreflector, and a version of the open-air SMR that has a window covering. Construction of an SMR starts with a solid sphere, which in most cases is made of stainless steel. SMRs are expected to be accurate and durable and it is the steel sphere that provides the contact surface for the measurement and protects the optics from damage during use. Different alloys of stainless steel are used to balance magnetic properties against corrosion resistance. Steel spheres are categorized into grades that describe their dimensional properties. A common ball grade for an SMR is Grade 25. The number 25 refers the sphericity in millionths of an inch (0.000025 inches). The other properties of the ball are also controlled by the grade specification. A Grade 25 ball specifies a surface roughness tolerance of no more than 0.000002 inches and a diameter tolerance of +/−0.0001 inches.
The heart of the SMR is the cube-corner retroreflector. In one type of SMR, three flat glass panels are bonded together to make the cube-corner retroreflector. Before the flat glass panels are assembled into cube corners, they are most often coated with protected silver. The panels are often matched to minimize polarization effects and reflectance variation. The three glass panels are bonded into an assembly, which is centered in the sphere. Glass-panel SMRs can provide some of the highest performance targets available. However, a weakness of glass-panel SMRs is the glass panels themselves, which are easily broken if dropped, subjected to an impact load, or just not handled carefully.
Through the common handling abuse that may occur during daily use, the adhesive bonding to the three glass panels can release the whole optical assembly, or a single panel within the assembly can shift from its nominal position. This can distort the beam and lead to errors in measurements.
A second type of SMR is the integrated optic SMR, which is a solid steel sphere where the retroreflector is machined directly into the sphere. Creating the three mutually perpendicular surfaces into a hardened sphere requires time and expensive processes such as electrical discharge machining (EDM) that lead to the higher costs. The optical reflective surfaces are transferred into the sphere through a process called replication. A replicated optic begins by coating a master with a release layer, a thin layer or gold, and a thin layer of epoxy. The machined metal is referred to as a substrate and is pressed onto the master and allowed to cure. The adhesive layer takes up any variation in the surface of the substrate leaving a precise copy of the master when removed from the tool. While this design represents the most break resistant and stable design, machining of the hardened steel has limited the possible accuracy. Unlike glass panels having surfaces that are stiff and flat, replicated optics have surfaces that are relatively soft and can be damaged through aggressive cleaning. Because the entire SMR is made almost entirely of steel, with only a thin epoxy layer, the integrated optic SMR design has proven to be very stable over extreme temperature changes.
A third type of SMR features a single replicated optic mounted into a hardened stainless steel sphere. The optic is manufactured in a replication process similar to that of the integrated optic SMR, with the difference being the material out of which the cube-corner optic is made. In place of the difficult and expensive to machine hardened steel, aluminum is used for the substrate. The single optic is an aluminum cylinder with the three mutually perpendicular faces machined and the gold reflective surfaces applied through replication. Because the aluminum-based optic is easier to manufacture it offers a middle ground in cost. The assembly process is similar to that of the glass panel SMR in that the retroreflector is centered in the sphere and secured by a high performance adhesive. This design allows for very precise centering, yielding an SMR with high accuracy.
The second type of SMR is inherently break resistant, and it is possible through careful design to also make the third type of SMR break resistant. Break-resistant SMRs may be dropped onto a hard floor without the vertex of the SMR moving in relation to the SMR spherical surface. To make the third type of SMR break resistant, the type and thickness of the adhesive layer must be carefully considered in light of the properties of the replicated slug and the spherical portion.
In view of the high degree of accuracy required of SMRs, it will be appreciated that the different SMR properties can significantly impact a laser tracker's ability to track and measure to the fullest of its capabilities. The stainless steel ball can contribute to measurement uncertainty if the sphericity or diameter is not known accurately or if it becomes worn and develops flat spots or areas where the diameter is not nominal. Radial measurement systems are susceptible to polarization errors in an improperly manufactured SMR. A common cause of polarization error is the uneven application of the protective coatings on protected silver retroreflectors. Most laser tracker systems are sensitive to polarization in one mode or another. If the SMR causes the polarization state to change and the IFM system requires a certain state, then the optical interference pattern may not be created clearly. Some laser trackers utilize a polarization modulation technology for their ADM that could be impacted by a changed polarization state of an SMR. Mirrors with poor reflectance from poor coatings or damaged optical surfaces will return a weak signal. In this case, the SMR may track poorly or, more importantly, the ADM or IFM system may have reduced measurement accuracy. The transverse measurement performance can be impacted by the SMR as described in ASME B89.4.19-2006, Appendix B. The B89 document discusses 3 types of SMR uncertainty contributions. The first two are mechanical properties related to the lateral and radial centering of the retroreflector in the sphere. The third property is related to dihedral angle errors. The dihedral angle error is the deviation in the angles of the adjacent panel from perpendicular. This deviation can cause measurement errors in trackers for the case in which a position sensitive detector (PSD) “retrace point” is not properly set. Laser trackers are compensated to establish the retrace position but this compensation is not perfect. Consequently, it is important that the SMR is manufactured to a specific dihedral angle tolerance and that these dihedral angles are maintained over use. An explanation of the condition in B89.4.19, Appendix B, is where one or two of the SMR panels have a high dihedral angle error in respect to the others. As a result, the optical center can be shifted and not represent the mechanical center of the retroreflector. The offset beam will cause the apparent center of the beam to change as the SMR is rotated in a nest. This type of error is called runout error and may be the result of either the cube corner within the sphere being off center or a dihedral angle error. However, the runout patterns have a different appearance when the cube corner is off center and dihedral angle error exists, as is explained in the B89.4.19 standard, Appendix B.
Another dihedral angle error occurs when all three panels are tilted into the center or away from the center. If the beam becomes expanded enough on the return, it can clip on the optics and cause the beam on the PSD not to be round (Gaussian) as is preferred.
Beyond the errors in the centering of the vertex with respect to the spherical surface, there are several other specifications that are significant to an SMR's performance. An SMR is supposed to return the laser beam to the tracker without added distortion. SMR induced errors can be the result of dihedral angle errors, as described above, or wave front distortion. Dihedral angle errors are generally reported with two values: total error and adjacent angle error.
Total error can cause the beam to expand or contract on the return path to the tracker. This may cause the beam shape to distort. Adjacent angle error, on the other hand, can lead to a shift in the optical center of the beam and produce optical runout when rotating the SMR. Wavefront distortion is a measure of the change in the wavefront shape as a result of reflection off the mirror panels of the SMR. It may be caused by panels that are not perfectly flat. When the laser beam is reflected off an SMR having panels that are not flat, the wavefront is altered from its original flat form. This can result in increased error in the systems of the tracker, including the IFM, ADM, and angle measuring systems. The term wavefront distortion refers to a composite measurement of distortion that includes effects due to panel flatness and dihedral angle errors since both effects influence the wavefront of the laser beam returning from the retroreflector. Within the reflective region of the SMR, the center of the target is the most critical as this is the area where the power of the laser beam is most concentrated.
A specification that quantifies the quality of the retroreflector in this critical region is called central wavefront distortion. As an example, this specification may consider wavefront quality over just the central 6 mm region of the cube corner.
To meet customer requirements, an SMR needs to maintain the required performance over the temperature range of the laser tracker and not be permanently altered at the even more extreme potential storage temperatures. The storage temperature range is typically −40° C. to 70° C., while the operating temperature range is typically −15° C. to 50° C. The target needs to be able to be subjected to these extreme storage temps and return to the in-tolerance specifications and geometry for the operational temperature range.
In addition to withstanding the above noted temperature ranges, to be considered break-resistant, the SMR must be capable of maintaining the optic in the proper position over the operational temperature range while withstanding at least 10 drops to a concrete floor from a standard operating height. At the same time, it has to be stiff enough to maintain the cube corner at the same position over time.
There are two types of mechanical deformation that can occur under strain of extreme temperature changes, with or without impact loading; elastic and plastic. Elastic deformation means that the geometry of the SMR may exceed tolerance at the ends of the storage temperature range but return to an in-tolerance condition within the operating temperature range. Plastic deformation means that the geometry of the SMR is permanently altered to an out-of-tolerance condition even when returned to ambient temperatures.
As explained hereinabove, an SMR is an extremely high precision instrument, where the vertex of a cube-corner retroreflector within an SMR is ideally placed at the exact center of the sphere into which the cube-corner is embedded. In practice, however, the position of the vertex is off the center of the sphere by up to a few thousandths of an inch. In many cases, the difference in the positions of the vertex and the sphere center are known to high accuracy, but for some SMR designs the position of the vertex relative to the sphere center may change significantly with temperature. Furthermore, this data may be rendered valueless if the SMR is subjected to extreme temperatures and/or is dropped or subjected to an impact force that displaces the vertex of the cube-corner retroreflector.
A particular difficulty encountered in the manufacture of SMRs is obtaining the very small value of sphericity ordinarily desired—for example, a sphericity of 25 millionths of an inch for a grade 25 ball. In one machining strategy, a cavity is machined into a sphere to provide a pocket for a retroreflector insert. In some cases, the sphericity of the ball may be degraded by machining the cavity. To avoid this potential degradation, another machining operation may be performed afterwards to obtain the desired sphericity. Ordinarily an SMR must be both very hard and non-corrosive. To obtain the desired corrosion resistance, a material such as SAE 440 stainless steel may be used. The selected material should have high corrosion resistance, easy machinability, suitable coefficient of thermal expansion (CTE), and relatively low cost. To also obtain the desired hardness of the SAE 440 stainless steel while maintaining the ability to machine the ball, the ball may be machined first and heat treated afterwards by raising the temperature to between 1010 and 1065° C. to harden the steel. A potential problem with this approach is that the heat treatment may degrade the sphericity of the sphere.
While existing SMRs may be suitable for their intended purpose, there remains, however, a need in the art for SMRs that are heavy duty, have improved break resistance over specified temperature ranges, maintain good centering of the vertex within the spherical surface of the SMR, have good sphericity, and are economical to fabricate.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.